The RcParams class has a mechanism for deprecating keywords, and aliasing to an alternative keyword with a way to convert its data into the new data. This is useful for setting values. However, it uses the same alternates mechanism for getting the keyword's data, but no way to invert the conversion.

For example, I plan to deprecate 'axes.color_cycle' for 'axes.prop_cycle'. The color_cycle is a list of strings, while a prop_cycle is a cycler object. The deprecation mechanism makes it easy for me to add a lambda of the form lambda x: cycler('color', x), so that setting a list to 'axes.color_cycle' still works. But if anyone did rcParams['axes.color_cycle'], they would get back a cycler object instead of a list of colors.